The present invention relates to the production of gypsum wallboard, and more specifically, to devices for modifying the flow of gypsum slurry upon a moving conveyor line.
It is well known to produce gypsum products (i.e., products comprising calcium sulfate dihydrate) from starting materials comprising calcined gypsum (i.e., calcium sulfate hemihydrate) and water. A popular application of gypsum chemistry is in the production of gypsum wallboard panels. The basic technology of gypsum wallboard panel manufacture is disclosed in U.S. Pat. Nos. 1,500,452; 2,207,339 and 4,009,062 all of which are incorporated by reference. In this process, calcined gypsum is uniformly dispersed in water to form a slurry, and then the slurry is deposited upon a continuously moving web of facing paper located on a conveyor line. After deposit upon the face paper, a top layer of backing paper is deposited upon the slurry, which is then cast into a desired shape and allowed to set to form hardened gypsum by reaction of the calcined gypsum (calcium sulfate hemihydrite or anhydrite) with the water to form hydrated gypsum (calcium sulfate dihydrate). As is well known in the art, after the panels are formed, they are heated to dry the excess water, and cut into building panels.
It is also well known to produce a lightweight gypsum product by uniformly mixing an aqueous foam into the slurry to produce air bubbles. This will result in a uniform distribution of voids in the set gypsum product if the bubbles do not escape from the slurry before the hardened gypsum forms. The voids lower the density of the final product, which is often referred to as “foamed gypsum.”
A gypsum wallboard mixer typically includes a housing defining a mixing chamber with inlets for receiving calcined gypsum and water, as well as other additives well known in the art, including but not limited to foam. The mixer usually includes an impeller or other type of rotating agitator for agitating the contents to be mixed into a mixture or slurry.
In some gypsum wallboard production techniques, a first, outer layer of relatively higher density slurry and less foam is first deposited upon the moving web of face paper, for creating a relatively durable outer region of the panel, and is then followed by a relatively lower density slurry formulated for creating the core of the wallboard panel. The higher density outer layer has been found to be more resistant to “nail pull,” which is the force needed to pull the wallboard panel over the head of a nail used to fasten the board to an underlying wooden frame. Also, in some cases, a last, backing layer of relatively high density slurry is then deposited upon the core layer.
It has been found that it is desirable to reduce the pressure of the slurry in the slurry conduit before the slurry leaves the conduit outlet in order to avoid disrupting the distribution of the previously deposited slurry in a wallboard production line. This is accomplished by providing one or more changes of direction of the conduit between the mixer and the conduit outlet, such as by providing one or more elbows or bends along the length of the conduit and also by enlarging a cross section of the flow stream of slurry in the conduit while at the same time changing the direction of the flow stream. In the known constructions, the enlargement of the flow stream and the changing of the direction of the flow stream take place simultaneously in a boot which forms a 90-degree elbow that has an increasing diameter throughout the 90-degree bend of the elbow. An exemplary wallboard mixer outlet boot is disclosed in commonly-assigned U.S. Pat. No. 8,475,762, which is incorporated by reference.
It has been found that, in many cases, even when using angled boots as described above, when the lower density core slurry is deposited upon the previously deposited, higher density slurry, at the point of impact of the later applied slurry upon the previously applied slurry, the denser slurry is subject to surface disruption, or “washout,” which results in unsatisfactory panels.
Thus, there is a need to reduce or eliminate the above-identified washout in the production of gypsum wallboard panels.